1. Field of the Invention
The present invention relates to ceramic substrates for mounting electronic components and the like thereon, and more particularly to a surface structure of a ceramics substrate having a superior heat resistance. A method of manufacturing such a substrate is also disclosed.
2. Background Information
A surface of a ceramics substrate is metallized to mount or seal electronic components. With reference to FIG. 1 showing a conventional surface structure of a ceramics substrate, a refractory metal 2 such as W or Mo is formed by baking on a ceramic substrate 1 of alumina, for example. An Ni plating layer 3 is formed on the metal 2, and an Au plating layer 4 is formed on the same layer 3. The Ni plating layer 3 facilitates soldering of the electronic components or the like, and serves to protect the metallized layer 2 formed thereunder. The Au plating layer 4 prevents oxidation of the Ni plating layer 3 formed thereunder to provide a superior connection between the Ni plating layer 3 and the electronic components, thereby maintaining by an excellent reliability of the electronic components and the like.
When elements are to be bonded on the Au plating by soldering, the ceramics substrate or the surface of the ceramics substrate is heated. However, the uppermost Au layer is tarnished by the heating, which prevents forming an excellent bonding. The cause of the tarnish of the Au plating layer may be diffusion caused by heating of Ni of the Ni plating layer into the Au plating layer whereby Ni is coupled with oxygen introduced from the air to form Ni oxide.
In order to avoid the above described problem, the thickness of the Au plating layer is generally made as thick as about 2 .mu.m. However, in order to reduce cost, the Au plating layer should preferably be reduced in thickness.
Japanese Patent Laying Open No. 59-114846 discloses a method of suppressing the diffusion of Ni from the Ni plating layer to the Au plating layer. More specifically, after the Ni plating layer is formed, the Ni plating layer is heat treated within a temperature range of 900.degree. C. to 1400.degree. C. in a reducing atmosphere, so as to improve the heat resistance of the Ni plating layer.
However, the above method cannot sufficiently prevent tarnishing of the Au plating layer. Especially when the thickness of the Au plating layer is not larger than 1 .mu.m, the Au plating layer is tarnished when electronic components and the like are bonded.